According to Japanese Patent Publication No. H4-355714A, an optical waveguide of an optical control element is turned back at an end face of a substrate, thereby extending an interaction length between a lightwave and a modulating wave, decreasing a drive voltage, compensating a matching velocity difference between the lightwave and a signal wave, and realizing a high-speed operation.
According to “Highly Functional Hybrid Modules Using Low Loss Direct Attachment Technique with Planar Lightwave Circuit and LiNbO3 Devices”, T. Yamada et al., ISMOT-2005, pp. 107-110, 2005, a planar optical circuit (PLC) and an LN waveguide device are connected to each other. Moreover, Japanese Patent Publication No. 2005-173162A describes a connection between a PLC and an LN waveguide, and, on this occasion, there is a difference between mode field diameters of the optical waveguides on two substrates therefore, and there is thus provided a tapered portion for adjusting the difference in the mode filed diameter.
It should be noted that Japanese Patent Publication No. 2004-245991A describes a structure of using adhered fixtures at both chip ends.